This invention relates to a color cathode ray tube and more particularly to a color cathode ray tube panel having an improved panel pin mounting construction for supporting a shadow mask.
Generally, in a color cathode ray tube, panel pins for supporting a shadow mask are fixed to a panel. Normally, the panel pins are fixed on the skirt of the panel. In fixing on the panel skirt, there are two methods: one is that the panel pins are fixed embedded at about the center points of four inner side sections of the skirt and the other is that the panel pins are fixed embedded at four corner sections of the skirt as disclosed in U.S. Pat. No. 4,652,792.
As described in U.S. Pat. No. 4,652,792, in the latter fixing method, a mask frame of a substantially rectangular shadow mask is supported, through elastic support members, by the panel pins at four corners and the mask frame is subject to less deformation than in the former fixing method in which the mask frame is supported at the center portions of its four sides. Hence, the latter method provides less chances for electron beams to be mislanded on the screen, particularly at the corner areas in completed cathode ray tubes.
Since a mask frame of a substantially rectangular shadow mask is supported, through elastic support members, by panel pins at four corners, the shadow mask is less liable to vibrate when an external impact is given to the shadow mask, thus reducing variations in beam landing caused by vibration. Mask frames used for the latter fixing method, having stability against vibration, can be made thinner than mask frames adopted for the former method.
As disclosed in U.S. Pat. No. 4,652,792, it is possible to correct, without using a bimetal that is conventionally installed in a cathode ray tube, a long time purity drift phenomenon which appears more than 30 minutes after the cathode ray tube is energized.
It has been pointed out, however, that the fixing method, in which panel pins are embedded at four corner sections and which has various advantages described above, involves the following problems.
Generally, panel pins are integral bodies, each consisting of a taper section connected with an elastic support member and a cylindrical embedded section to be embedded in a corner of a panel, leaving the taper section projecting. When such a panel pin is embedded at corner, the embedded section cannot be embedded in the corner section to a uniform depth around its periphery because the inner surface of the corner section is curved. The result is an insufficient embedded strength of panel pins.
With cathode ray tubes named FS tube (tradename) which are disclosed in U.S. Pat. Nos. 4,537,321, 4,537,322 and 4,535,907 and Ser. No. 844,553 filed on Mar. 28, 1986, the curvature of the corner sections of the panel is large in every case, resulting in low mounting strength and low embedding accuracy of the panel pins. In large-size cathode ray tubes, the weight of the shadow mask is so great as to liable to impose a large load on the panel pins and therefore, the panel pins are required to have a high mounting strength.
With the construction in which panel pins are embedded at the corner sections of a panel, however, a sufficient mounting strength cannot be obtained for the panel pins. Therefore, there is a possibility that a crack occurs at the corners where panel pins are embedded, due to a thermal shock applied to the panel or a mechanical shock in mounting or dismounting a shadow mask during the manufacturing process of cathode ray tubes.
Since panel pins are embedded in curved corner sections, four panel pins cannot be positioned with accuracy so high as in embedding in flat skirt sections. Panel pins tend to be mounted off specified positions or a specified angle. If the panel pins are mounted with poor accuracy, a shadow mask cannot be held in correct position by panel pins. Consequently, phosphor stripes, which are formed by exposure through apertures of a shadow mask, cannot be arranged in correct position, deteriorating color reproducibility or making color adjustment difficult in a completed color cathode ray tube. In a structure in which the panel pins are embedded at the corner sections such that the panel pins are incorrectly inclined or positioned, the embedded depth is more uneven around their peripheries and therefore, the embedded strength is more reduced.
Generally, panels are molded by pressing molten glass in a mold. In the press molding process, a plunger that presses molten glass is pulled out of the mold before the molten glass solidifies, that is to say, an unsolidified glass panel comes out of the mold, thus entailing a possibility that the skirt of the glass panel inclines slightly either inside or outside. This slight inclination of the skirt is liable to deteriorate the mounting accuracy. To avoid this, delicate adjustments are required.
For example, if the skirt of a glass panel is inclined slightly inside, panel pins need to be embedded in the skirt deeper than normally in order to secure specified diagonal dimensions between two pairs of diagonally opposite panel pins. Similarly, if the skirt of a glass panel is inclined slightly outside, panel pins need to be embedded in the skirt shallower than normally in order to secure specified diagonal dimensions between two pairs of diagonally opposite panel pins.
When the skirt of a glass panel is inclined slightly inside, panel pins are embedded in the skirt deeper than normally, resulting in glass rising along the peripheries of the panel pins when the molten glass solidifies and offering a possibility that the risen glass having a strain is scarred by elastic support members fixed to a shadow mask when the shadow mask is fitted to the panel pins or detached from the panel pins. If a glass panel is scarred, cracks may develop in the glass panel due to a thermal shock applied during the manufacturing process of color cathode ray tubes.
When the skirt of a glass panel is inclined slightly outside, panel pins are embedded in the skirt shallower than normally, reducing the mounting strength of panel pins and increasing a possibility of cracks developing in the glass panel by a thermal or mechanical shock.
When panel pins are mounted at corners of a panel, a phosphor slurry, which is introduced into the panel to form a phosphor screen, is likely to adhere to the panel pins. If this happens, the deposited phosphor will fall off and adhere to electrodes of an electron gun, for example, in the tube, thus deteriorating the dielectric strength during the manufacturing process or after completion of the cathode ray tube.
A phosphor screen is made as follows. A glass panel is rotated, a phosphor slurry is introduced to the substantially central portion of the rotating panel, the phosphor slurry is spread along the inner surface of the faceplate by the use of centrifugal force and excess phosphor slurry that has reached the skirt is collected from the skirt. In this process, the excess phosphor slurry collects at the corners of the panel where the centrifugal force acts greatest and is discharged from the corners to the outside of the panel.
In FS type color cathode ray tubes with a rectangular screen, which are the dominant type in the market, the skirt is sharp-cornered, making phosphor slurry concentrate at the corners and increasing the possibility of its adhering to the panel pins. The phosphor slurry adhering to the panel pins deteriorates the repeatability of shadow mask mounting operations and, if it falls off, decreases the dielectric strength of the color cathode ray tube.
As described above, in color cathode ray tubes for which panel pins are mounted at corners of a skirt of a substantially rectangular panel and a shadow mask is installed between the panel pins by connecting elastic support members to the panel pins, the basic problem is that the embedding depth of a panel pin is uneven around its periphery. Therefore, the mounting strength of panel pins is lower than in ordinary color cathode ray tubes in which panel pins are embedded at center points of four sides of the skirt. Worse still, the mounting accuracy decreases.
In addition, when panel pins are installed off specified mounting positions or angle and the skirt is inclined outside by press molding, the mounting accuracy is reduced still lower. As color cathode ray tubes become larger, the shadow mask increases in weight, increasing a load on the panel pins. When the load on the panel pins increases, there are increased chances for cracks to occur around the panel pins by a thermal or mechanical shock during the manufacturing process of color cathode ray tubes.
When the skirt is inclined in press molding, a rise of glass around the panel pins increases. As a result, the risen portions are scratched when they are contacted by the elastic support members in mounting and dismounting the shadow mask. The scratches may lead to the occurrence of cracks by a thermal shock applied in the manufacturing process of color cathode ray tubes. Even if scratches or cracks are minute, stresses and resulting strains occur in a cathode ray tube, particularly at the corners when the air is purged from the tube and the tube is subjected to atmospheric pressure. Then, the minute cracks will develop and result in an implosion when worst comes to worst.
With panels in which panel pins are mounted at the corners of the skirt, excess phosphor slurry is discharged from the corners when a phosphor screen is formed. The phosphor which adheres to the panel pins impairs the repeatability of shadow mask mounting operations. Later, the deposited phosphor comes off and deteriorates the dielectric strength of the color cathode ray tube.